1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof, and more particularly to what is suitable for an in-plane type liquid crystal display device embodying an attempt to reduce the overall cost by curtailing the formation step of a common transparent electroconductive film in the in-plane system.
2. Description of the Related Art
Liquid crystal display devices are extensively used as typical flat panel displays (FPDs). Basic structures of a liquid crystal display device can be broadly classified into two systems. One is the so-called twisted nematic system (commonly known as the TN system) in which a pixel electrode is formed over one of two substrates and a common electrode (counter electrode or common electrode) is formed over the other to form an electric field between the two electrodes and the alignment of liquid crystal molecules is controlled. The other is the so-called in-plane system (commonly known as IPS) in which a pixel electrode and a common electrode (counter electrode) are formed on one of two substrates and substantially in parallel to the surface of the substrate and the alignment of liquid crystal molecules is controlled with an electric field formed between the two electrodes. Further, various derivatives from these systems have been proposed and put into actual use.
Among liquid crystal display devices of these systems, the in-plane system has such advantages as a wide field of view and a high production yield due to the configuration in which both the pixel electrode and the common electrode on one substrate (usually they are formed on the substrate on which a thin film transistor is formed: the thin film transistor substrate or TFT substrate) to allow a greater tolerance of alignment with the other substrate (which usually is configured as a color filter substrate (CF substrate) by forming color filters).
In a liquid crystal display device of the in-plane system, an effective display area in which a large number of pixel circuits made up of thin film transistors are arranged in a matrix and external circuits including a driving circuit for controlling the turning on/off of pixels outside this effective display area are installed or formed on the main face of the TFT substrate, which is one of the substrates of the system. On the other hand, color filters of three colors including R (red), G (green) and B (blue) are usually formed on the main face of the CF substrate, and the CF substrate and the TFT substrate are stuck together with their main faces meeting each other, with liquid crystals being sealed between the stuck faces. The present invention relates to an in-plane type liquid crystal display device, in particular to the common electrodes provided on the TFT substrate side and the formation thereof. Although color filters are arranged on the TFT substrate side in some cases, no detailed description will be made of them herein because the invention is not relevant to color filters.
FIG. 1 shows a section of one pixel to illustrate a structural example of a conventional TFT substrate. For this TFT substrate, an inorganic substrate of glass or the like or an organic film of heat resistant plastic or the like is used, and a thin film transistor, a pixel electrode and a common electrode are formed on its main face (the inner face to configure the TFT and so on). In the following description, the use of a transparent glass substrate is supposed. Referring to FIG. 1, a TFT part TFT has a laminated structure including a metal film 3 (of aluminum (Al) or some other metal) over the main face of a glass substrate 1. In a pixel aperture part PX, there is an ITO film as a common electrode 2 on the same layer as gate electrode/gate wiring 3. In a common wiring part (a power feed part for the common electrode) CL, an upper common electrode 2A is stacked over the metal film (of aluminum (Al) or some other metal) over the ITO film forming the common electrode 2.
A gate insulating film 4 is formed over the metal film 3 the ITO film as the common electrode 2 and the upper common electrode 2A. Silicon nitride (SiN) is suitable as this gate insulating film 4, but some other appropriate insulator can be used as well. In the layer over the gate insulating film 4, a semiconductor film 5 is patterned in the TFT part TFT. This semiconductor film 5 has a laminated structure including a layer 5B of amorphous silicon, polysilicon or the like and a doped semiconductor layer 5A over it.
A source electrode 6 and a drain electrode 7 are formed over the semiconductor film 5 with a channel in-between. These source electrode 6 and drain electrode 7 are formed by patterning metal, preferably aluminum, films. A protective insulating film 8 is formed covering the gate insulating film 4 including these source electrode 6 and drain electrode 7 over it. A suitable material for the protective insulating film 8, which may also be called a passivation film (PAS film), is silicon nitride as for the gate insulating film 4. However, some other appropriate insulator can be used as well. Incidentally, though the source electrode 6 and the drain electrode 7 are switched over when in operation, they are fixed in the illustration for the convenience of description.
A pixel electrode 9 made up of ITO is patterned over the protective insulating film 8. The pixel electrode 9 is comb-shaped, and part of it is connected to the drain electrode 7 of TFT through a contact hole bored in the protective insulating film 8. Further in a common wiring part CL, common wiring 11 is connected to the upper common electrode 2A via a through hole bored in the protective insulating film 8 and the gate insulating film 4.
Over the top layer of the main face of this TFT substrate, an alignment film is formed to provide an alignment control function by rubbing or otherwise. After that, the CF substrate is stuck to this TFT substrate, and liquid crystals are sealed in between them to structure a liquid crystal display panel. External circuits including a driving circuit chip are packaged around the TFT substrate constituting the liquid crystal display panel or built into the substrate face. Mechanical parts including a backlight, a print substrate mounted with a display signal control circuit and the like are incorporated into this liquid crystal display panel to assemble a liquid crystal display device.
FIG. 2 illustrates the process of forming the common electrode shown in FIG. 1. In FIG. 2, P-1 represents the sputtering step of an ITO film; P-2 represents the photolithographic step of the ITO film; P-3 represents the etching step of the ITO film; P-4 represents the sputtering step of a metal film; P-5 represents the photolithographic step of the metal film; and P-6 represents the etching step of the metal film. FIGS. 3A to 3F show sections of the TFT substrate matching the steps of processing illustrated in FIG. 2. FIG. 4 is a plan of the TFT substrate after the ITO etching shown in FIGS. 3A to 3F. FIG. 5 is a plan of the TFT substrate after metal etching shown in FIGS. 3A to 3F.
The formation process of the common electrode will be described below with reference to FIG. 2, FIGS. 3A to 3F, FIG. 4 and FIG. 5. First, at the step denoted as P-1 in FIG. 2 (hereinafter to be denoted simply as P-1 and so forth), an ITO film 20 is formed on the main face of a glass substrate 1 (FIG. 3A). Photosensitive resist 21 is applied onto the film, and is patterned as a common electrode at the photolithographic step (denoted simply as “Photo” in the drawing) (P-2, FIG. 3B). After etching the ITO film 20 with the patterned photosensitive resist 21 used as the mask, the photosensitive resist 21 is removed (P-3, FIG. 3C and FIG. 4). FIG. 4 shows the planar shape of the etched ITO pattern. The section along line A-B in FIG. 4 corresponds to FIG. 3C. Aluminum (Al) 30 is sputtered as a metal film over the photosensitive resist 21 (P-4, FIG. 3D) and, after applying photosensitive resist 41, the resist 41 is left in the parts of a gate wiring/electrode 3 and the upper common electrode wiring (common wiring) 2A by masked exposure (P-5, FIG. 3E). This structure is then etched to remove the metal elsewhere than underneath the resist 41 to expose the common electrode 2 (P-6, FIG. 3F and FIG. 5).
Thus, the conventional practice requires two photolithographic steps, one for ITO patterning and the other for the patterning of the gate wiring/electrode metal. Incidentally, a technique by which some or all of the insulating film, electroconductive film and semiconductor film constituting a TFT are formed by direct patterning by ink jet application, in place of sputtered film formation and photolithographic steps in TFT substrate manufacture, is disclosed in Japanese Patent No. 3725169 cited in connection with the present patent application.
Cost saving is a factor of vital importance to liquid crystal display panels constituting liquid crystal display devices for large TV sets. Since patterning by sputtered film formation and photolithography require expensive and large items of manufacturing equipment including vacuum devices for sputtering, exposure masks and developing devices, reductions in sputtered film formation and photolithography could directly contribute to reductions in manufacturing cost.
Direct patterning of thin films by ink jet application disclosed in Japanese Patent No. 3725169 is claimed to achieve patterning accuracy to the order of 10 μm. As the size of one pixel of a liquid crystal display panel for TV use is relatively large, about 500 μm×170 μm, theoretically the pixel electrode and other elements seem to be directly formed by ink jet application. However, the ink for use in ink jetting is a solution in which a thin film material is dispersed, and therefore ink drips dropped from the nozzle wet-spreads over the surface onto which the drips have fallen, making it difficult to accurately control the outer periphery of the applied edge.
In trying to reduce the number of steps of sputtered film formation and photolithography for manufacturing liquid crystal display panels of a type in which a comb-shaped pixel electrode having common electrodes, one solidly formed for each pixel, is disposed over the main face of a glass substrate via an insulating film, if the common electrode greater in square measure than the pixel electrode can be formed by ink jet application, a significant contribution can be made to cost saving. However, to prevent short-circuiting between wiring lines and inter-layer stray conductance, the patterning of the common electrodes should be accurate to 5 μm or even below, and no such accurate patterning can be achieved by the ink jet application according to Japanese Patent No. 3725169.